Medical imaging of internal organs provides important anatomic and diagnostic information, which medical personnel can employ to make therapeutic decisions. Medical images can be acquired using various non-invasive imaging procedures, such as computed topography (CT), magnetic resonance imaging (MRI), or ultrasound imaging. A CT system transmits x-rays through an anatomic site of interest, and based on the attenuation coefficients of the x-rays, cross-sectional images (“slices”) of the object can be reconstructed. As a result, the CT system is well suited for viewing details about bony structures, diagnosing diseases of the lung and chest, and detecting cancers. Advantages of CT imaging also include, for example, a short scan time for a complete scan (typically less than five minutes), low cost (about half the price of an MRI apparatus), the ability to accurately outline bony tissue inside the body, fewer motion artifacts due to fast imaging speeds (each scan time is less than 30 seconds), etc. CT systems, however, irradiate the patient and pose consequent risk; for this reason, CT scans are not recommended for pregnant women or children.
Ultrasound imaging, which involves passing high-frequency acoustic waves through the body, is another widely used technique. Ultrasound waves penetrate well through soft tissues and, due to their short wavelengths, can be focused to spots with dimensions of a few millimeters. In a typical ultrasound examination, a transducer probe is placed directly on the skin or inside a body opening. A thin layer of gel may be applied to the skin to provide direct contact between the transducer probe and skin and thereby allow efficient transfer of ultrasound energy into the body. An ultrasound image of internal anatomic structures can be constructed from the waves reflected by those structures—in particular, for example, from the amplitudes and phases of the reflection signals and/or the time it takes for the ultrasound waves to travel through the body. Because ultrasound images are captured in real-time, they can also show movement of the body's internal organs as well as blood flowing through the blood vessels. In addition, ultrasound imaging offers high temporal resolution, high sensitivity to acoustic scatterers (such as calcifications and gas bubbles), excellent visualization, low cost, portability, and no ionizing radiation exposure, and is thus generally considered safe even for pregnant women and children.
MRI is still another imaging modality that is used to visualize the target tissue; MRI can be used in conjunction with ultrasound to guide the ultrasound focus during therapy as further described below. In brief, MRI involves placing a patient into a homogeneous static magnetic field, thus aligning the spins of hydrogen nuclei in the tissue. Then, by applying a radio-frequency (RF) electromagnetic pulse of the right frequency (the “resonance frequency”), the spins may be flipped, temporarily destroying the alignment and inducing a response signal. Different tissues produce different response signals, resulting in a contrast among these tissues in MR images. Because the resonance frequency and the frequency of the response signal depend on the magnetic field strength, the origin and frequency of the response signal can be controlled by superposing magnetic gradient fields onto the homogeneous field to render the field strength dependent on position. By using time-varying gradient fields, MRI “scans” of the tissue can be obtained.
Many MRI protocols utilize time-dependent gradients in two or three mutually perpendicular directions. The relative strengths and timing of the gradient fields and RF pulses are specified in a pulse sequence. Time-dependent magnetic field gradients may be exploited, in combination with the tissue dependence of the MRI response signal, to visualize, for example, a brain tumor, and determine its location relative to the patient's skull. MRI has advantages including multi-planar imaging capability (without moving the patient), high signal-to-noise ratio, high sensitivity to subtle changes in soft tissue morphology and function, and no radiation exposure. But MRI suffers from its sensitivity to patient movement due to the long scan time (typically between 30 minutes to a few hours), lower-resolution images of bony structures, and interference from the operation of other radio-frequency (RF) devices.
Because each imaging technique has its own strengths and weaknesses and may provide different types of information, it may be advantageous in practice to combine different imaging techniques. For example, combining a CT scan with MRI can provide good details about bony structures as well as subtle differences between soft tissues. A combination of MRI and ultrasound imaging has been shown to provide additional diagnostic information for better diagnosis in intraoperative neurosurgical applications and breast biopsy guidance. Further, because ultrasound energy can be employed therapeutically—e.g., to heat and ablate diseased (e.g., cancerous) tissue without causing significant damage to surrounding healthy tissue—the combination of MRI and ultrasound provides imaging capability during therapeutic medical procedures. An ultrasound focusing system generally includes an acoustic transducer surface, or an array of transducer surfaces, to generate one or more ultrasound beams. In transducer arrays, the individual surfaces, or “elements,” are typically individually controllable—i.e., their vibration phases and/or amplitudes can be set independently of one another—allowing the beam to be steered in a desired direction and focused at a desired distance. The ultrasound system often also includes receiving elements, integrated into the transducer array or provided in form of a separate detector, that help monitor ultrasound-based treatment. For example, the receiving elements may detect ultrasound reflected by interfaces between the transducer and the target tissue, which may result from air bubbles on the skin that need to be removed to avoid skin burns. The receiving elements may also be used to detect cavitation in overheated tissues (i.e., the formation of cavities due to the collapse of bubbles formed in the liquid of the tissue).
A focused ultrasound transducer system may include MR tracking coils or other markers for determining the transducer position and orientation relative to the target tissue (such as a tumor) in the MR image. Based on computations of the required transducer element phases and amplitudes, the transducer array is then driven so as to focus ultrasound at the target. The ultrasound focus itself may be visualized using MRI or acoustic resonance force imaging (ARFI), and such visualization may be used to adjust the focus position. These methods are generally referred to as magnetic-resonance-guided focusing of ultrasound (MRgFUS).
To successfully integrate two or more imaging systems and/or combine information provided by distinct systems, it is necessary to register image data that may be obtained in different imaging coordinate systems. Conventional approaches to registration typically involve complex computational procedures and may not provide sufficient accuracy for therapeutic purposes. For this reason, utilizing such image registration without verification may result in an inaccurate application of energy and unsuccessful or lengthy treatment. Accordingly, there is a need for establishing and verifying registration of images obtained using different modalities in a sufficiently fast, reliable manner to support therapeutic applications.